When analysing the wetting behaviour of small samples with finely structured surfaces, the drops used for measurement must be small, too. To enable measurements on these structures, DataPhysics Instruments has introduced the FDDS FlexDrop dosing system for their contact angle meters of the OCA series, allowing reliable contact angle and surface energy measurements on extremely small structures.

Matching drop size to structure size

The contact angle is only meaningful when the drop used to measure it sits on the surface area of interest – and only this area. When the relevant area – a screw thread, a strut, a bond pad – is itself less than a millimetre wide, a conventionally dosed drop in the microlitre range already floods over the edge of the surface area of interest. The result is a misleading test value, as it does not reflect the wetting behaviour of the material under investigation. For many small components, this means working with drops in the nanolitre or even picolitre range is necessary. The size of the drops used for contact angle measurements must be chosen with care. Using an unnecessarily small drop only adds complexity to the measurement. Therefore, it is important to use the appropriate size for every surface structure (Figure 2).

• Conventional dosing, such as syringe-based dosing, with a minimum drop volume around one microlitre: to be used on flat, homogeneous, macroscopic samples such as sheet material, coated panels, or bulk polymer parts.
• Nanolitre dosing with volumes down to two nanolitres: moderately structured surfaces with sub-millimetre features, such as the screw threads of an implant, fine mesh, or densely populated circuit boards.
• Picolitre dosing with volumes down to 25 picolitres: to be used on the smallest individual features – such as stent struts, individual bond pads, or micromechanical components.

How to dose tiny drops

Dosing extremely tiny drops is not possible with conventional syringe-based dosing systems used for contact angle measurements. DataPhysics Instruments has developed the solution: the FDDS FlexDrop dosing system, a module for the contact angle meters of the manufacturer’s OCA series. It is purpose-built for dosing small drops with pinpoint accuracy and reproducibility. The FDDS FlexDrop can be fitted with two different dosing heads, one for nanolitre- and one for picolitre-sized drops. With this technology, drops with as little as 25 picolitres in volume can be generated.

When analysing small samples with tiny drops, it is important that the camera of the measurement device has a high optical resolution, so that the drops are recognised adequately by the software. For example, the contact angle meter OCA 200 combines a powerful 10-fold zoom lens with an autofocus system. Hitting such small targets reliably also depends on a precise positioning of the sample. One example is electrically driven sample tables, which can be moved in a highly precise fashion.

Advantages of small drops

Nanolitre and picolitre drops have a practical advantage: because of their small volume, they often evaporate within seconds. Almost no residue is left on the sample. For high-value or delicate components this is especially advantageous. It means the component can be measured, as part of quality control, and then returned to further use.

Unlike a syringe-based dosing system, the FDDS FlexDrop dosing system uses a liquid cartridge to store the test liquid. The cartridge can be replaced with another in a few simple steps. Therefore, there is no risk of cross-contamination when several liquids are used in sequence.

The modular approach of the OCA series has another benefit: standard drops as well as drops in the nano- and picolitre range can all be realised with a single contact angle meter from the OCA series. By exchanging only the dosing unit of the system, a laboratory can move from routine macroscopic samples to small sample areas using the same instrument.

One-click surface energy determination

Determining the surface energy of a solid requires measurements with at least two test liquids of known surface tension. Water and diiodomethane are frequently used liquids for surface energy analysis. On a normal sample, dosing two liquids one after another is straightforward. On a surface of only a few square millimetres, this approach becomes impractical. The FDDS FlexDrop dosing system solves this by carrying up to two dosing heads at the same time. Both test liquids are deposited simultaneously, side by side, with only one millimetre in between. The device software, dpiMAX, then calculates the surface energy automatically.

Application examples across industries

The need for small, precisely placed drops arises wherever components are miniaturised or their surfaces are finely structured. This is the case in many industries:

• Precision mechanics and watchmaking: The wetting and spreading of lubricants on gear teeth, pivots, and jewel bearings affects both friction and long-term wear. The relevant components are only a few square millimetres in size. The FDDS FlexDrop with two dosing heads allows the surface energy of such components to be determined with a single click (Figure 3).
• Electronics manufacturing: Wetting and cleanliness of individual solder pads and bond sites determine the reliability of a solder joint. Small drops allow these properties to be checked pad by pad on a fully populated board, without wetting adjacent components (Figure 4).
• Medical technology: Wettability is closely linked to how well tissue or bone integrates with an implant, for example, a dental screw implant. The same applies to the wire mesh of a coronary stent, whose struts are far too narrow for a conventional drop, and catheters, which are only a few millimetres in diameter.
• Printing processes: A fully automated OCA 200 contact angle meter with an FDDS FlexDrop picolitre dosing system can simulate printing routines like those of an inkjet printer. When developing functional silver inks for conductive tracks, the simulation of such an experiment can mimic real-life printing during development of a new ink formula. Using the easily exchangeable cartridges of the FDDS rather than a printer, possible damage to expensive printer heads can be avoided.

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